(1) Field of the Invention
The present invention relates to a semiconductor laser manufacturing method, and in particular to a technology for preventing the COD (Catastrophic Optical Damage) level from being lowered.
(2) Description of the Related Art
In manufacturing high-output semiconductor lasers, what is called an asymmetric coating is generally adopted to coat end faces of a resonator.
With the asymmetric coating, a low reflecting coating is formed on an end face of the resonator through which laser beams are emitted, and a high reflecting coating is formed on the other end face of the resonator, for the purpose of emitting laser beams efficiently.
In a preparative stage for the asymmetric coating, what is called a plasma cleaning is performed to expose cleavage planes of a semiconductor laminated structure, which are to be the end faces of the resonator, to inert gas (such as argon gas) in plasma state for several minutes. This removes from the cleavage planes substances that had been floating in the air before attaching to the cleavage planes when the semiconductor laminated structure was cleaved in the air. Dirty cleavage planes decrease the degree of contact of the coating. It is therefore desirable that the cleavage planes are clean enough.
Meanwhile, the inventors of the present invention found through experiments that with conventional technologies for plasma cleaning, the Catastrophic Optical Damage (COD) level is lowered with the extension of the duration of the plasma cleaning.
It should be noted here that the COD indicates a phenomenon in which the heat of laser light emitted from an end face of a resonator of a semiconductor laser is absorbed into the end face of the resonator to destroy the end face, and that the COD level indicates the lowest output value of the laser light at which the COD is generated.
The following describes the experiments conducted by the inventors of the present invention.
First prepared were a GaAs substrate, which is the experimental sample, and an Electron Cyclotron Resonance (hereinafter ECR) sputtering apparatus.
FIG. 19 is a cross-sectional view of the ECR sputtering apparatus.
In FIG. 19, the ECR sputtering apparatus 5 includes: a plasma chamber 64 in which an ECR plasma gas is generated; a coating forming chamber 60; a silicon target 61 having degree of purity not lower than 5N; and coils 66 provided around the plasma chamber 64 to form magnetic fields.
In the plasma chamber 64, a micro wave, which is introduced into the chamber through a quartz window, and magnetic fields formed by the coils 66 put a process gas, which is introduced into the chamber through a gas introduction pipe 67, into the ECR plasma state.
The silicon target 61, which is connected to an RF power supply 65, controls the sputtering yield by changing the applying voltage in largeness.
The coating forming chamber 60 is connected to an exhaust system (not illustrated) via an exhaust opening 68. With this construction, it is possible to reduce the pressure in the coating forming chamber 60.
A sample board 62 with a GaAs substrate 63 set thereon is placed in the coating forming chamber 60 to form a coating on the GaAs substrate 63. In this case, the amount of energy of the process gas in the plasma state that reaches the surface of the GaAs substrate 63 is determined by the ECR divergence magnetic field distribution.
For example, when the sample board 62 is 20 cm away from the ECR point, the amount of energy of the process gas in the plasma state is approximately 10–20 eV. It should be noted here that these values are obtained when no voltage is applied to the sample board 62.
The samples were subjected to experiments under the conditions (i) and (ii) shown in Table 1, respectively.
TABLE 1Plasma cleaningConditionsPlasma cleaning gasduration(i)No plasma cleaning (reference)(ii)Argon gas (30 sccm)10 minutes
The plasma cleaning was performed under conditions (ii) as follows.    (1) The internal pressure during the gas introduction was approximately 10−1 Pa. The strength of the micro wave was approximately 600 W.    (2) To restrict the amount of the sputtering occurring at the silicon target 61 to as small a degree as possible, no voltage was applied to the silicon target, and the sample board 62 was set to a normal temperature.    (3) After the plasma cleaning, the samples were extracted from the ECR sputtering apparatus 5, without forming coatings on the samples.
The surfaces of the GaAs substrates of the samples for each of the conditions (i) and (ii) were analyzed using the Auger electron spectroscopy. The following Table 2 shows the results of the analysis.
TABLE 2ConditionsPlasma cleaning gasDetected elements(i)No plasma cleaningCarbon, oxygen(ii)Argon gas (30 sccm)Silicon, oxygen
The analysis results shown in Table 2 provide the following findings.    (1) Carbon was detected on the surface of the GaAs substrate 63 of the samples that were not subjected to a plasma cleaning (condition (i)). This shows the effect of the plasma cleaning since carbon was not found from the samples that were subjected to the plasma cleaning.    (2) Silicon was detected on the surface of the GaAs substrate 63 of the samples that were subjected to the plasma cleaning using only the argon gas (condition (ii)). This shows that a silicon film is formed on the surface of the GaAs substrate 63. The amount of the silicon film, when converted from the thickness of the coating, is presumed to be approximately 10 Å. That is to say, it is estimated that a silicon film of approximately 10 Å is formed on an end face of the resonator for a semiconductor laser when a conventional plasma cleaning is performed under the condition (ii). This means that after the coating is formed later, the silicon film remains between the coating and the end face of the resonator.
It is estimated that the silicon film is provided from the silicon target 61. Until this experiment was conducted, it had been considered that the sputtering is caused only when a negative voltage of not lower than 100V is applied to the silicon target 61. It was confirmed through this experiment, however, that a small amount of sputtering occurs at the surface of the silicon target 61 during a plasma cleaning.
It is considered that this phenomenon occurs for the following reason. As shown in FIG. 20, after a plasma discharge starts, a potential of approximately −5V to −10V is generated as the sheath potential of plasma. This causes a potential difference on the surface of the silicon target 61.
Ions in the process gas in the plasma state are drawn toward and collide with the silicon target 61. This causes the sputtering at the surface of the silicon target 61, allowing ions of silicon to emit from the silicon target 61. The ions of silicon reach the sample (GaAs substrate 63) and are heaped up on the surface thereof.
The silicon film, which is presumed to be formed as described above, is considered to be a significant factor in lowering the COD level. This is because, for example, when an amorphous silicon film is formed on an end face of the resonator of an AlGaInP base semiconductor laser, the amorphous silicon film absorbs the light having the laser emission wavelength (630–680 nm).